MW 1.23+ / Hotfix for empty ParserOutput caused by RefreshLinksJob

For details on the issue, see [0] as well as the method to replicate
the problem.

[0] https://github.com/SemanticMediaWiki/SemanticMediaWiki/issues/347#issuecomment-48595047
In the process where `isEmpty` is caught (as in the above case) the new parse would not run through `NewRevisionFromEditComplete`  (which is responsible to add `_MDAT` early on) therefore is missing the pre-defined property but successively added by by the StoreUpdater which still counts for the final 4 properties ( 2 user-defined + '_MDAT' and '_SKEY).